Batrachotoxinin-A-20-Alpha-p(3H)benzoate ((3H)BTX-B) is a potent analog of batrachotoxin which exhibits a selective action on voltage-sensitive sodium channels in a variety of excitable tissues resulting in a decrease of the resting membrane potential. We have found that (3H)BTX-B binds to sodium channels of synaptosomes with an affinity and specificity sufficient to permit further application of this compound as a m olecular probe of sodium channel function. Our study proposes to monitor the effects of pH, membrane potential, and chemical modification at the sodium channel on the specific binding of (3H)BTX-B to synaptosomes. The results should provide information relevant to the biochemical nature of the (3H)BTX-B binding site and potentially to the conformational changes of the sodium channel protein which accompany channel activation and inactivation. Furthermore, we plan to pursue a synthetic program directed towards the development of fluorescent and radioactive photoactivatable derivatives of batrachotoxin. These new derivatives will be valuable in conjunction with ongoing studies concerning the relative spatial locations and microenvironments of neurotoxins bound t the sodium channel as well as the isolation, purfication and subunit composition of this important integral protein. These studies should help provide a better understanding of voltage sensitive sodium channels in particular and the mechanisms of membrane excitability generally.